Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, a gas chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2-) move toward the anode through the electrolyte. A fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) or CO is supplied to the anode. The oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating DC electric energy.
Power generation reaction (H2+½O2→H2O) of the solid oxide fuel cell is exothermic reaction. Therefore, the temperature of the solid oxide fuel cell is significantly high. Fuel reforming reaction (e.g., in the case of methane, CH4+2H2O→CO2+4H2) in steam reforming of hydrocarbon by a reformer is endothermic reaction. Therefore, it is desirable to substantially match the heat generation distribution in the power generation reaction in the cell and heat absorption distribution by steam reforming reaction. In this regard, for example, a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2003-317785 is known.
In the conventional technique, as shown in FIG. 14, a solid oxide fuel cell 3 is formed by stacking reformers 1 and cells 2 alternately. The reformers 1 perform steam reforming by supplying a hydrocarbon fuel gas to reforming catalyst such as nickel-based catalyst. Each of the cells 2 includes a fuel electrode 2A, an air electrode 2C, and an electrolyte layer 2B interposed between the fuel electrode 2A and the air electrode 2C.
Each of the reformers 1 has hollow structure for forming a catalyst filling passage 4. That is, the reforming catalyst is filled in the hollow part of the reformer 1. Further, a fuel electrode side passage 5 as a passage of a reforming gas is formed on the upper surface of the reformer 1, and an air electrode side passage 6 is formed on the lower surface of the reformer 1.
A reforming gas inlet 5a of the fuel electrode side passage 5 is provided adjacent to a hydrocarbon fuel gas inlet 4a of the catalyst filling passage 4. A reforming gas outlet 4b of the catalyst filling passage 4 is connected to the reforming gas inlet 5a of the fuel electrode side passage 5 through a return passage 7.
In the conventional technique, the reformers 1 and the cells 2 are stacked together. The catalyst filling passage 4 and the fuel electrode side passage 5 are provided adjacent to each other. In the structure, the reforming gas outlet 4b of the catalyst filling passage 4 and the reforming gas inlet 5a of the fuel electrode side passage 5 are connected through the return passage 7. Thus, the solid oxide fuel cell 3 has complicated structure. The overall width of the solid oxide fuel cell 3 is significantly large.